Despite the rapid developments in manufacturing processes (and in particular methods of building on a rigid core) in recent years, the majority of tires today are still built on drums. In this type of process, the carcass reinforcement, formed of one or more plies, is laid on a cylindrical drum such that its reinforcement elements are oriented in a direction substantially parallel to the axis of the drum. The carcass reinforcement having been wound around the drum, its ends are assembled such that the reinforcement forms a cylinder, the internal diameter of which corresponds to the diameter of the drum. The ends are assembled by means of a junction zone making it possible to connect the two edges of the reinforcement.
Two types of junction are commonly used: end-to-end junctions and overlapping junctions.
In the end-to-end junction of the edges of a ply, the connection of the ply edges depends on the adhesiveness of small portions of rubber composition coating the reinforcement elements. The junction zone is therefore not very strong and may deteriorate during the transformation of the cylindrical blank of the reinforcement into a toroidal blank during the later shaping of the tire, in particular in the zones of greater circumferential development.
It has been proposed for a long time to reinforce the end-to-end junction zone by applying a small layer of rubber, located close to the zone. Thus, document U.S. Pat. No. 1,899,067 proposes laying a sheet of adhesive rubber on the edges of the carcass reinforcement and applying heat and pressure to ‘weld’ the zone. Document EP 0 120 623 discloses a more recent variant of the same approach. FR 2 389 483 proposes replacing the rubber with a reinforced cover strip, the reinforcing threads of which are oriented virtually along a circumference of the cylinder formed by the carcass reinforcement. Document DE 2 504 181 pursues this reasoning by proposing using a pre-stretched strip, which makes it possible to supply more resistance to the zone during the transformation of the cylindrical blank of the reinforcement into a toroidal blank, and to avoid the formation of zones of great thickness (imbalance).
As its name indicates, the overlapping junction consists of covering, over a certain distance, an edge of a ply of the carcass reinforcement with the other edge and connecting them by application of pressure. This type of junction is more particularly used when the fabric is reinforced by textile cords or cables. It is particularly robust, due to the large surface of adhesion between the overlapping parts.
Having a double thickness of fabric over a certain circumferential distance, this junction is however not without its drawbacks. The number of reinforcement elements is doubled over the junction zone; under the action of the inflation pressure, the elongation of the reinforcement elements of the zone is therefore twice as little compared with the elongation of all the other reinforcement elements of the reinforcement. The result is a loss of uniformity of the sidewalls of the tire, which may have an influence on the performance of the tire or at the very least on its appearance.
Various solutions have been proposed: EP 0 407 134 discloses the use of a rubber cover strip to cover all or part of the junction zone. U.S. Pat. No. 4,810,317 teaches using a reinforced strip covering the entire junction zone, the reinforcement elements of the strip being arranged preferably at an angle of 10 to 25 degrees relative to the reinforcement elements of the carcass reinforcement.
These solutions do not always permit satisfactory reduction of the irregularity of the sidewall. They also have the drawback of requiring the addition of a relatively large quantity of rubber or of rubberized fabric and of increasing the imbalance of the tire. The addition of a reinforced strip covering the entire junction zone may even have an effect contrary to the desired aim, because this strip can act as a reinforcement of the carcass ply in the junction zone, thus making worse the deformation of the sidewall when the tire is inflated.
Other solutions have been proposed. It has in particular been suggested to cut the reinforcement elements in that part of the ply where there is superposition. This solution effectively makes it possible to reduce the zone of greatest resistance to elongation, but it is a delicate operation and involves the risk that too many elements will be cut and that fragile zones will thus be formed.
It has also been proposed to provide a carcass reinforcement comprising reinforcing elements of lesser resistance to elongation in the junction zone. This solution constitutes a solution to the problem of the deformation of the sidewall, but it is expensive and difficult to implement on an industrial scale.
The first object of the present invention is to solve the problem of the irregularity of the sidewall by simple means, industrial use of which is not costly.